Methods and systems for computing notional source signatures from near-field measurements and modeled notional signatures
Methods and systems for computing notional source signatures from modeled notional signatures and measured near-field signatures are described. Modeled near-field signatures are calculated from the modeled notional signatures. Low weights are assigned to parts of a source pressure wavefield spectrum where signatures are less reliable and higher weights are assigned to parts of the source pressure wavefield spectrum where signatures are more reliable. The part of the spectrum where both sets of signatures are reliable can be used for quality control and for comparing the measured near-field signatures to modeled near-field signatures. When there are uncertainties in the input parameters to the modeling, the input parameters can be scaled to minimize the differences between measured and modeled near-field signatures. Resultant near-field signatures are computed by a weighted summation of the modeled and measured near-field signatures, and notional source signatures are calculated from the resultant near-field signatures.
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In the past few decades, the petroleum industry has invested heavily in the development of marine seismic survey techniques that yield knowledge of subterranean formations beneath a body of water in order to find and extract valuable mineral resources, such as oil. High-resolution seismic images of a subterranean formation are essential for quantitative seismic interpretation and improved reservoir monitoring. For a typical marine seismic survey, an exploration-seismology vessel tows a seismic source and one or more streamers that form a seismic data acquisition surface below the surface of the water and over a subterranean formation to be surveyed for mineral deposits. The vessel contains seismic acquisition equipment, such as navigation control, seismic source control, seismic receiver control, and recording equipment. The seismic source control causes the seismic source, which is typically an array of source elements, such as air guns, to produce acoustic impulses at selected times. Each impulse is a sound wave that travels down through the water and into the subterranean formation. At each interface between different types of rock, a portion of the sound wave is refracted, a portion of the sound wave is transmitted, and another portion is reflected back toward the body of water to propagate toward the surface. The streamers towed behind the vessel are elongated cable-like structures. Each streamer includes a number of seismic receivers or sensors that detect pressure and/or velocity wavefields associated with the sound waves reflected back into the water from the subterranean formation.
In order to process seismic data measured at the acquisition surface to produce focused seismic images of a subterranean formation, accurate knowledge of a pressure wavefield created by the seismic source is desired. However, obtaining an accurate characterization of the source pressure wavefield is often met with difficulty. For example, the source pressure wavefield can be determined from pressure measurements taken within near fields of the source elements, but the measurements can be contaminated with noise caused by cross-talk and from the hydrophones picking up some of the motion caused by firing other powerful source elements in the vicinity of the hydrophone. Other techniques to accurately characterize the source pressure wavefield include modeling the source pressure wavefield. The models are typically calibrated with actual measurements taken at far-field distances from the source elements and rely on a number of input parameters, such as positions of the source elements, pressures, and water temperature. Predominant errors in source wavefield modeling are typically related to the accuracy of the calibration and the assumptions made in modeling. As a result, those working in the petroleum industry continue to seek systems and methods to more accurately characterize the source pressure wavefield.
Methods and systems for computing notional source signatures from modeled notional signatures and measured near-field signatures are described. Modeled near-field signatures are calculated from the modeled notional signatures. Weights as a function of frequency are determined from comparisons between the modeled near-field signatures and the measured near-field signatures in the frequency domain. Low weights are assigned to parts of the source pressure wavefield spectrum where the signatures are less reliable and higher weights are assigned to parts of the source pressure wavefield spectrum where the signatures are more reliable. The part of the spectrum where both sets of signatures are reliable can be used for quality control and for comparing the measured near-field signatures to modeled near-field signatures. When there are uncertainties in the sensitivity of the near-field hydrophones, the modeling can verify and determine the sensitivities of near-field hydrophones. When there are uncertainties in the input parameters to the modeling, the input parameters can be scaled to minimize the differences between measured and modeled near-field signatures. Resultant near-field signatures are computed by a weighted summation of the modeled and measured near-field signatures and notional signatures are calculated from the resultant near-field signatures.
The following discussion includes two subsections: an overview of exploration seismology; and a description of a method for computing notional source signatures from near-field measurements and modeled notional signatures as an example of computational processing methods and systems to which the current disclosure is directed. Reading of the first subsection can be omitted by those familiar with exploration seismology.
An Overview of Exploration SeismologyAs shown in
Acoustic and elastic waves, however, travel at different velocities within different materials as well as within the same material under different pressures. Therefore, the travel times of the initial pressure impulse and secondary waves emitted in response to the initial pressure impulse are complex functions of distance from the acoustic source as well as the materials and physical characteristics of the materials through which the acoustic wave corresponding to the initial pressure impulse travels. In addition, as shown in
The acoustic source 312 can be implemented as an array of seismic source elements, such as air guns and/or water guns, in order to amplify sound waves and overcome undesirable aspects of a signature associated with using a single source element.
Each gun has an associated near-field signature and a far-field signature. “Near field” and “far field” are terms used to describe proximity of an observation point to a gun when the signature is measured. For a gun that releases a pressure wave with a wavelength λ=c/f, where c is the speed of sound in the fluid, and f is the frequency, the near-field and far-field radial regions surrounding the gun can be defined as:
Near field: d<λ
Intermediate field: d˜λ
Far field: λ<<d
where d is the distance from the gun to an observation point
As shown in
The detailed features of a signature are determined by the subsequent motion of the bubble following its release from a gun.
The guns of a gun array are selected with different chamber volumes and arranged in a particular manner in order to generate a resulting far-field seismic wave with a short and narrow signature in the vertical-downward direction and with a spectrum that is smooth and broad over a frequency band of interest.
Note that acoustic sources are not intended to be limited to the example thirty-three gun array 510 shown in
Methods and systems for computing notional signatures from near-field measurements and modeled notional signatures are now described.
In block 602, a recorded near-field signatures, pjrec(t), obtained from a pressure measurement at the jth pressure sensor is input, where t represents time. In block 603, modeled notional source signatures, p′i(t), associated with each source element of the acoustic source are input. A “notional” source signature is an isolated near-field signature of the pressure wavefield near the i-th source element with pressure wavefields created by other neighboring source elements removed and reflections from the free surface removed. The distances and locations of the pressure sensors and source elements of the acoustic source are known and can be used to calculate the modeled notional source signatures p′i(t) associated with each of the source elements using a seismic analysis and data processing techniques, such as those techniques provided in Nucleus+(see e.g. http://www.pgs.com/pageFolders/308427/NucleusplusBrochureOctober2010.pdf) and described in “The growth or collapse of a spherical bubble in a viscous compressible liquid,” by F. R. Gilmore, Office of Naval Research, Report No. 26-4, Apr. 1, 1952. In block 604, a modeled near-field signature associated with the jth source element is calculated from the modeled notional source signatures p′i(t) as follows:
where rij is the distance from the ith source element to the jth pressure sensor or near-field measurement position;
r′ij is the total distance along a ray path from the ith source element up to the free surface and down to the jth pressure sensor or near-field measurement position;
R is the reflection coefficient of the free surface;
c is the propagation velocity of pressure waves in the fluid; and
n is the number of near-field pressure sensors and the number source elements.
Returning to
where
-
- β=0, 1, 2, . . . , N−1;
- N is the number of time samples;
- ωβ is the βth angular frequency sample; and
- tα is the sample time.
And the modeled near-field signature can be transformed using a discrete Fourier transform given by:
In practice, the recorded near-field signature and the modeled near-field signature can be transformed using a fast Fourier transform for computation efficiency.
Returning to
Pj(ω)=W(ω)sjPjrec(ω)+[1−W(ω)]Pjmod(ω) (4)
where sj is a scale factor computed below in block 609; and
W(ω) is a weight function to transition from the recorded near-field signature pjrec (ω) to the modeled near-field signature pjmod(ω) as a function of the frequency ω. The weight function W(ω) has the properties given by:
W(ω)=1 for 0<ω≤ω1.
0<W(ω)<1 for ω1<ω<ω2
W(ω)=0 for ω2≤ω
An example of a suitable weight function is described below with reference to block 610. In block 607, rather than using visual inspection to compare the recorded near-field signature to the modeled near-field signature, as described above with reference to
where
prrpjrec(ω)
pmm=pjmod(ω)pjmod(ω) (5b)
prm=pjrec(ω)
The spectral coherence Crm is a fractional value that ranges between “0” and “1” and can be used as a metric to determine the degree to which the recorded and modeled near-field signatures are correlated, with “0” indicating no correlation and “1” indicated a strong correlation.
Returning to
-
- where ω0<ω<ω1 is the frequency range over which the recorded near-field signature pjrec(ω) and the modeled near-field signature pjmod(ω) have the highest spectral coherence, as described above with reference to
FIG. 10 .
When calibration of the pressure sensors is known, the scale factor sj is used to correct the unit of the measured near-field signature (e.g. mV) to a pressure unit (e.g. Pa). Also, in this case, the measured near-field signatures can be used to calibrate the modeled near-field signatures, unless the measured and modeled near-field signatures are in agreement. In block 610, the weight function for ω1<ω<ω2 can be calculated using, for example, the Hanning weight function given by:
- where ω0<ω<ω1 is the frequency range over which the recorded near-field signature pjrec(ω) and the modeled near-field signature pjmod(ω) have the highest spectral coherence, as described above with reference to
where ƒ(ω) is function.
When the function ƒ(ω) ranges between “−1” and “0” the weight function W(ω) ranges between “0” and “1,” and when the function ƒ(ω) ranges between “0” and “1” the weight function W(ω) ranges between “1” and “0.” For example, the function ƒ(ω) can be a linear function given by:
where ω1<ω<ω2.
In this example, the function ranges from “0” to “1” as the frequency ω is increased from ω1 to ω2. Alternatively, different types of functions ƒ(ω) and weight functions W(ω) can be used to control the influence the recorded near-field signature or the modeled near-field signature have over the range of frequencies ω1<ω2. In block 611, the near-field signature associated with the jth source element is computed according to Equation (4).
where α=0, 1, 2, . . . , N−1; and
-
- Pi(ωβ) is given by Equation (4).
In practice, an inverse fast Fourier transform can be used for computational efficiency. In block 613, when more source elements are available, the operations associated with blocks 602-612 are repeated until a resultant near-field signature pj(t) has been computed for each of the n source elements. Otherwise, the method proceeds to block 614. In block 614, n notional source signatures are calculated from the near-field signatures pj(t) by solving a set of n equations with n unknowns given by:
where pj(t) is the resultant near-field signature computed in block 612; and
-
- p′i(t) are n unknown notional source signatures associated with each of the n pressure sensors.
The n notional signatures p′i(t) can be computed from Equation (10) iteratively in time steps. The secondary contributions from the surrounding source elements and the associated ghosts are subtracted from the near-field signature to derive each notional source signature. At a time t, the notional source signatures from the surrounding guns p′i at times
- p′i(t) are n unknown notional source signatures associated with each of the n pressure sensors.
have already been calculated in an earlier time step, because
and is already known. This method relies on the number of near-field pressure sensors being the same as the number of source elements.
Although the present invention has been described in terms of particular embodiments, it is not intended that the invention be limited to these embodiments. Modifications within the spirit of the invention will be apparent to those skilled in the art. For example, any number of different computational-processing-method implementations that carry out efficient computation of notional source signatures using modeled notional source signatures and measured near-field signatures may be designed and developed using various different programming languages and computer platforms and by varying different implementation parameters, including control structures, variables, data structures, modular organization, and other such parameters. The computational representations of wavefields, operators, and other computational objects may be implemented in different ways.
It is appreciated that the previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present disclosure. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the disclosure. Thus, the present disclosure is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims
1. In a process for generating an image of a subterranean formation using marine seismic techniques in which a source is activated above the subterranean formation and the reflections from the subterranean formation are recorded as seismic data generated by receivers and a near-field signature of each pressure wave generated by a source element of the source is captured in a recording generated by a pressure sensor located within a near field of each source element, the specific improvement comprising:
- generating a modeled near-field signature for each source element, wherein the modeled near-field signature of a source element is a combination of modeled notional source signatures of each source element;
- combining a low-frequency portion of the recorded near-field signature with a high-frequency portion of the modeled near-field signature of each source element to generate a new recording corresponding to a new near-field signature of each source element, wherein the new near-field signature of each source element avoids an unreliable high-frequency portion of the recorded near-field signature and an unreliable low-frequency portion of the modeled near-field signature;
- computing notional source signatures of the source elements using the new near-field signatures, each notional source signature characterizing an isolated pressure wave generated by a source element without effects of pressure waves generated by other source elements and without free surface reflections;
- determining a source pressure wavefield from the notional source signatures; and
- generating an image of the subterranean formation based on the source pressure wavefield and the seismic data.
2. The process of claim 1, wherein the source elements are air guns selected with chamber volumes, air gun spacings, and positions within in the source to dampen bubble oscillations of the pressure wave generated by each of the source elements.
3. The process of claim 1, wherein the number of source elements equals the number of pressure sensors.
4. The process of claim 1, further comprising:
- comparing the modeled near-field signature to the recorded near-field signature in a frequency domain using spectral coherence to determine a range of frequencies over which the recorded near-field signature and the modeled near-field signature are in agreement; and
- scaling the recorded near-field signature to the modeled near-field signature using the modeled and recorded near-field signatures over the range of frequencies, when calibration of pressure sensors of the acoustic source are unknown.
5. The process of claim 1, comprising:
- comparing the modeled near-field signature to the recorded near-field signature in a frequency domain using spectral coherence to determine a range of frequencies over which the recorded near-field signature and the modeled near-field signature are in agreement;
- converting the measured near-field signatures to a pressure unit; and
- scaling the modeled near-field signature to the recorded near-field signature using the modeled and recorded near-field signatures over the range of frequencies, when calibration of pressure sensors of the acoustic source are known.
6. The process of claim 1, comprising:
- transforming the modeled near-field signature from a time domain to a frequency domain; and
- transforming the recorded near-field signature from the time domain to the frequency domain.
7. The process of claim 1, wherein combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element comprises combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element in a frequency domain.
8. The process of claim 1, wherein combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element comprises summing the recorded near-field signature multiplied by a first weight function and a scale factor with the modeled near-field signature multiplied by a second weight function, the first weight function greater than the second weight function for small frequencies and the first weight function less than the second weight function for large frequencies.
9. The process of claim 1, wherein computing the notional source signatures of each of the source elements using the new near-field signatures of the source elements comprises transforming the new near-field signatures from a frequency domain to a time domain.
10. A computer system for computing an image of a subterranean formation from recorded seismic data collected in a marine seismic survey of the subterranean formation, the system comprising:
- one or more processors;
- one or more data-storage devices; and
- machine-readable instructions stored in the one or more data-storage devices that when executed using the one or more processors controls the system to carry out generating a modeled near-field signature for each source element of a source activated above the subterranean formation in a body of water, wherein the modeled near-field signature of each source element is a combination of modeled notional source signatures of each source element; combining a low-frequency portion of the recorded near-field signature with a high-frequency portion of the modeled near-field signature of each source element to generate a new recording corresponding to a new near-field signature of each source element; computing notional source signatures of the source elements using the new near-field signatures, each notional source signature characterizing an isolated pressure wave generated by a source element without effects of pressure waves generated by other source elements and without free surface reflections; determining a source pressure wavefield from the notional source signatures; and generating an image of the subterranean formation based on the source pressure wavefield and seismic data generated by receivers that detect reflections from the subterranean formation in response to activation of the source.
11. The system of claim 10, wherein the source elements are air guns selected with chamber volumes, air gun spacings, and positions within in the source to dampen bubble oscillations of the pressure wave generated by each of the source elements.
12. The system of claim 10, wherein the number of source elements equals the number of pressure sensors.
13. The system of claim 10, further comprising:
- comparing the modeled near-field signature to the recorded near-field signature in a frequency domain using spectral coherence to determine a range of frequencies over which the recorded near-field signature and the modeled near-field signature are in agreement; and
- scaling the recorded near-field signature to the modeled near-field signature using the modeled and recorded near-field signatures over the range of frequencies, when calibration of pressure sensors of the acoustic source are unknown.
14. The system of claim 10, comprising:
- comparing the modeled near-field signature to the recorded near-field signature in a frequency domain using spectral coherence to determine a range of frequencies over which the recorded near-field signature and the modeled near-field signature are in agreement;
- converting the measured near-field signatures to a pressure unit; and
- scaling the modeled near-field signature to the recorded near-field signature using the modeled and recorded near-field signatures over the range of frequencies, when calibration of pressure sensors of the acoustic source are known.
15. The system of claim 10, comprising:
- transforming the modeled near-field signature from a time domain to a frequency domain; and
- transforming the recorded near-field signature from the time domain to the frequency domain.
16. The system of claim 10, wherein combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element comprises combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element in a frequency domain.
17. The system of claim 10, wherein combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element comprises summing the recorded near-field signature multiplied by a first weight function and a scale factor with the modeled near-field signature multiplied by a second weight function, the first weight function greater than the second weight function for small frequencies and the first weight function less than the second weight function for large frequencies.
18. The system of claim 10, wherein computing the notional source signatures of each of the source elements using the new near-field signatures of the source elements comprises transforming the new near-field signatures from a frequency domain to a time domain.
19. An apparatus for generating an image of a subterranean formation from recorded seismic data collected in a marine seismic survey of the subterranean formation and a near-field signature of each pressure wave generated by a source element of the source and recorded by a pressure sensor located within a near field of each source element, the apparatus comprising:
- means for generating a modeled near-field signature for each source element, wherein the modeled near-field signature of a source element is a combination of modeled notional source signatures of each source element;
- means for combining a low-frequency portion of the recorded near-field signature with a high-frequency portion of the modeled near-field signature of each source element to generate a new recording corresponding to a new near-field signature of each source element;
- means for computing notional source signatures of the source elements using the new near-field signatures, each notional source signature characterizing an isolated pressure wave generated by a source element without effects of pressure waves generated by other source elements and without free surface reflections;
- means for determining a source pressure wavefield from the notional source signatures; and
- means for generating an image of the subterranean formation based on the source pressure wavefield and the seismic data.
20. The system of claim 10, wherein the source elements are air guns selected with chamber volumes, air gun spacings, and positions within in the source to dampen bubble oscillations of the pressure wave generated by each of the source elements.
21. The system of claim 10, wherein the number of source elements equals the number of pressure sensors.
22. The system of claim 10, further comprising:
- means for comparing the modeled near-field signature to the recorded near-field signature in a frequency domain using spectral coherence to determine a range of frequencies over which the recorded near-field signature and the modeled near-field signature are in agreement; and
- means for scaling the recorded near-field signature to the modeled near-field signature using the modeled and recorded near-field signatures over the range of frequencies, when calibration of pressure sensors of the acoustic source are unknown.
23. The system of claim 10, comprising:
- means for comparing the modeled near-field signature to the recorded near-field signature in a frequency domain using spectral coherence to determine a range of frequencies over which the recorded near-field signature and the modeled near-field signature are in agreement;
- means for converting the measured near-field signatures to a pressure unit; and
- means for scaling the modeled near-field signature to the recorded near-field signature using the modeled and recorded near-field signatures over the range of frequencies, when calibration of pressure sensors of the acoustic source are known.
24. The system of claim 10, comprising:
- means for transforming the modeled near-field signature from a time domain to a frequency domain; and
- means for transforming the recorded near-field signature from the time domain to the frequency domain.
25. The system of claim 10, wherein the means for combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element combines the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element in a frequency domain.
26. The system of claim 10, wherein the means for combining the low-frequency portion of the recorded near-field signature with the high-frequency portion of the modeled near-field signature of each source element sums the recorded near-field signature multiplied by a first weight function and a scale factor with the modeled near-field signature multiplied by a second weight function, the first weight function greater than the second weight function for small frequencies and the first weight function less than the second weight function for large frequencies.
27. The system of claim 10, wherein the means for computing the notional source signatures of each of the source elements using the new near-field signatures of the source elements transforms the new near-field signatures from a frequency domain to a time domain.
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Type: Grant
Filed: May 30, 2012
Date of Patent: Mar 26, 2019
Patent Publication Number: 20130325427
Assignee: PGS Geophysical AS (Oslo)
Inventors: Stian Hegna (Hovik), Fabien Julliard (Oslo)
Primary Examiner: Dwin M Craig
Application Number: 13/483,327
International Classification: G06F 17/50 (20060101); G01V 1/13 (20060101); G01V 1/00 (20060101);